FIG. 1 illustrates the network architecture of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which is consisted of evolved Node Bs (eNBs). There is a control plane interface (X2-C interface) between adjacent eNBs, and when a User Equipment (UE) is handed over, a source eNB and a target eNB may further create a user plane interface (X2-U interface) for the UE to forward data.
A Mobility Management Entity (MME) is connected with an eNB via an S1-MME interface; and the eNB functions as an access network and communicates with the UE via an air interface. For each UE attached to the network, there is an MME serving the UE, which is referred as to a serving MME of the UE. The S1-MME interface provides the UE with a control plane service, including mobility management and bearer management functions.
A Serving Gateway (S-GW) is connected with an eNB via an S1-U interface. For each UE attached to the network, there is an S-GW serving the UE, which is referred to as a serving SGW of the UE. The S1-U interface provides the UE with a user plane service, and user plane data of the UE is transmitted between the S-GW and the eNB by a General Packet Radio Service (GPRS) Tunneling Protocol (GTP) of the S1-U interface.
In the network architecture of the E-UTRAN, the UE is connected with the same eNB in both the control plane and the user plane of the air interface. If the UE needs to be handed over from the source eNB to the target eNB, then the source eNB needs to configure the UE with measurement configuration information, and the source eNB makes a handover decision according to a measurement result reported by the UE. If the source eNB decides a handover to be performed, then it transmits a Handover Request message to the target eNB; the target eNB performs an admission decision according to Qualities of Service (QoS) of bearers to be admitted and other information, and when allowed admission of the UE is decided, the target eNB performs underlying configuration to prepare for the handover and returns to the source eNB a Handover Request Acknowledge message including a Radio Resource Control (RRC) container which contains a Handover Command to trigger the UE to be handed over, the bearer accepted by the target eNB to forward uplink/downlink data, and a transport layer address and a Tunnel Endpoint Identifier (TEID) of a forward tunnel; after the source eNB forwards the Handover Command to the UE, the UE stops the data from being received and transmitted at the source eNB; and the source eNB transmits to the target eNB, sequence number status information of the currently transmitted data (e.g., the sequence number of a downlink data packet which is transmitted unsuccessfully, the first sequence number which can be allocated by the target eNB, etc.), which further includes the identifier allocated previously by the protocol for the UE (e.g., an X2AP ID, an X2 interface application layer identifier, etc.) so that the target eNB can identify the UE to which the received sequence number is directed. The source eNB can further transmit a downlink data packet, which is received from a core network but has not been transmitted to the UE, to the target eNB so that the target eNB transmits it to the UE. The source eNB can further transmit an uplink data packet of the UE, received via the air interface, with a inconsecutive sequence number, to the target eNB so that the target eNB transmits the data packet with a consecutive sequence number to the core network upon reception of the missing data packet retransmitted by the UE; the UE transmits a preamble to the target eNB for uplink synchronization with the target eNB, the target eNB allocates an uplink resource, and a timing advance of the UE, for the UE, and the UE returns a Handover Complete message to the target eNB, so that the data can be transmitted and received between them; and thereafter the target eNB transmits a Path Switch Request to the MME, the MME further transmits a user plane transport layer address and a downlink GTP tunnel endpoint identifier, allocated by the target eNB for an Evolved Packet System (EPS) bearer of the respective UE, to the SGW, the SGW switches a downlink data transmission path of the UE to the target eNB, the SGW returns a Bearer Modify Response to the MME, and the MME switches both the user plane and the control plane of the UE to the target eNB after returning a Path Switch Response to the target eNB, that is, the UE sets up user plane and control plane radio bearers with the target eNB at a Uu interface, thus completing the path switching procedure, so that the source eNB can release the relevant resource allocated for the UE.
In the existing layered network as illustrated in FIG. 2, a macro cell provides underlying coverage, and a local cell provides hotspot coverage, and there is a data/signaling interface (a wired/wireless interface) between the local cell and the macro cell, and the UE can operate while being served by a macro eNB or a local eNB. Due to small coverage of by the cell controlled by the local eNB and a small number of UEs served by the cell controlled by the local eNB, the UE connected with the local eNB tends to be provided with a higher quality of service, e.g., a higher traffic rate, a link with a higher quality, etc. However due to a large number of local eNBs with small coverage, the UE being moved may be switched frequently between the cell controlled by the macro eNB and the cell controlled by the local eNB. The handover frequency and handover times may be increased significantly, thus resulting in a greater risk of interrupted communication of the UE being handed over.